1. Field of the Invention
The present invention relates to an optical scanner and an image forming apparatus.
2. Description of the Related Art
In a digital copier, a laser printer, a laser facsimile, and the like, an image is written using an optical scanner. Such an optical scanner includes a light source having a light emitter, a first optical system that forms an image of a light beam output from the light source as a long linear image extending in a main-scanning direction, a deflector having a deflecting reflective surface disposed near a position where the linear image is formed to deflect a light beam output from the first optical system, and a second optical system that condenses a light beam deflected by the deflector to a spot of light on a surface to be scanned, so that the surface is scanned with the light beam. A so-called multi-beam optical scanner in which a surface to be scanned is scanned with a plurality of light beams by using a multi-beam light source having a plurality of light emitters is also well known.
An increasing number of molded plastic products have come to be used for optical elements in the optical scanner, especially as a lens (scanning lens) used in the second optical system because the molded plastic products are economical and a free form surface can be achieved relatively easily. A molded plastic scanning lens is also positively adopted in the multi-beam optical scanner, in the same manner as in the conventional optical scanner having a single-beam light source.
In a molded plastic scanning lens, the refractive index distribution tends to be uneven.
Therefore, Japanese Patent No. 3518765, for example, discloses an optical element used in a multi-beam optical scanner that optically scans a target surface with a plurality of light beams and that includes a multi-beam light source having a plurality of light-emitting points, a first optical system that forms images with the light beams output from the light source as a plurality of long linear images extending in the main-scanning direction, an optical deflector having a deflective reflecting surface that deflects the light beams at a position near where the linear images are imaged, and a second optical system that condenses the deflected light beams into a plurality of light spots on the target surface. Such an optical element has an uneven refractive index distribution, and is a lens used in the second optical system. In this lens, a predetermined relationship is established between the number of multiple incident beams, the pitch of the principal rays of the multiple beams on the plane of incidence along the sub-scanning direction, the refractive index distribution, and the effective range in the sub-scanning direction corresponding to the effective write width of the target surface scanned by the light spots.
In addition, Japanese Patent Application Laid-open No. 2009-3393 discloses an optical scanner including a light source in which a plurality of light-emitting elements capable of performing optical modulation independently are arranged in the sub-scanning direction, a coupling optical element that converts the light beams output from the light-emitting elements into approximately parallel light beams, an aperture that defines the outer edge of the parallel light beams, a collimating optical element that collimates the form of the parallel beams in the sub-scanning direction, a deflecting element that deflects the collimated light beams for scanning, and a scanning optical system that forms images of the deflected scanning light beams to scan the target surface, in the order listed in this sentence. In this optical scanner, the scanning optical system includes a plurality of lenses including a resin lens having a positive power along the sub-scanning direction, and the aperture and the resin lens are in an optical conjugate relationship in the sub-scanning direction.
During a plastic molding process of an optical element, birefringence appears in a lens depending on its material, production conditions, its form, and other factors. Birefringence is a phenomenon where the refractive index becomes different for rays of light in directions perpendicular to each other, and is expressed by a main axis orientation and a phase difference. The main axis orientation herein has the same meaning as a fast axis orientation or a slow axis orientation.
Many scanning lenses are larger in size than pickup lenses (objective lenses), for example, used in an optical disk apparatus, and some molded plastic scanning lenses have an uneven birefringence distribution. In particular, a larger difference in thickness between the center and the peripherals of a lens, that is, a greater difference in thickness leads to more uneven birefringence distribution.
For example, it is assumed herein that, as illustrated in FIG. 40, two light beams (a beam 1 and a beam 2) output from different light emitters (ch1 and ch2) and separated from each other in the sub-scanning direction, pass through a scanning lens having a birefringence distribution illustrated in FIGS. 39A to 39C. In such a system, the birefringence of the scanning lens affects the beam 1 and the beam 2 differently. Therefore, as in an example illustrated in FIG. 41, the beam 1 and the beam 2, both of which are polarized linearly before being incident on the scanning lens, are polarized in a different manner after passing through the scanning lens. In FIG. 41, the beam 1 is elliptically polarized in a vertically elongated manner, and the beam 2 is elliptically polarized in a horizontally elongated manner. If a folding mirror is disposed between the scanning lens and the scanned surface, for example, because the reflectance of the beam 1 and that of the beam 2 are different on the folding mirror, the amounts of light on the scanned surface become different between ch1 and ch2. If the amounts of light on the scanned surface are different depending on the light emitters, the concentration of an output image might become uneven, and especially, banding might occur.
Furthermore, in a vertical cavity surface emitting laser array having a plurality of light emitters each outputting linearly polarized light, the direction of the polarity is rotated depending on the strength of the oscillation of the laser. The degree of the rotation differs in each of the light emitters (for example, see the paragraph 0003 in Japanese Patent Application Laid-open No. 2007-147864). If a vertical cavity surface emitting laser array having the light emitters outputting light with different polarization angles is affected by an uneven birefringence distribution, the difference in the amounts of light on the scanned surface is further increased, and the concentration of the output image would be more uneven, or banding would be more prominent.
Effects of an uneven refractive index distribution and an uneven birefringence distribution will now be explained. If a refractive index distribution is uneven, because the refractive power becomes different depending on a beam path, the light emitters have different focal points on the scanned surface, disadvantageously. This occurs only within a refractive optical system having a multi-beam light source and a scanning lens having an uneven refractive index distribution.
If the birefringence distribution is uneven, because the degree of birefringence becomes different depending on a beam path, the reflectance of the folding mirror and the transmittance of a dust preventing sheet glass, both of which are disposed between the molded plastic scanning lens and the scanned surface, become different for each of the beams. As a result, the amount of exposure on the scanned surface would be different for each of the light emitters, disadvantageously. This is caused by a multi-beam light source, a scanning lens having such an uneven birefringence distribution, and optical components (an optical reflecting member or an optical transmitting member) disposed between the scanning lens having an uneven birefringence distribution and the scanned surface. Such a system causes no difference in the amount of light on the scanned surface if no optical components are disposed between the scanning lens having an uneven birefringence distribution and the scanned surface.
As explained above, the effect of uneven birefringence distribution is different in quality from the effect of uneven refractive index distribution. Furthermore, the effect of the uneven birefringence distribution is not considered in the optical scanners disclosed in Japanese Patent No. 3518765 and Japanese Patent Application Laid-open No. 2009-3393.
In a vertical cavity surface emitting laser (hereinafter, also referred to as a “VCSEL”), it is known that a noise is generated when returning light enters the active layer. The returning light is light output from the light-emitting elements and reflected outside to return. Without any noise, a predetermined optical output can be obtained from an input current. However, when a noise is generated, the pulse waveform of the optical output is disturbed to change the amount of light emission. In such a case, a predetermined output cannot be obtained from an input current, and that results in an uneven density of the image.
The noise becomes larger when the wavelength of the returning light is closer to the wavelength of the light wave multiply reflected inside the resonator. The effect of the returning light in light beams output from an edge-emitting semiconductor laser (LD) is not so significant because a plurality of light beams having adjacent wavelengths are mixed in the light beams output from the LD as illustrated in FIG. 42 as an example.
On the contrary, in the light beams output from a VCSEL, the wavelength tends to exit in singularity as illustrated as an example in FIG. 43. This is because the wavelength of the resonator in the VCSEL is short, e.g., several wavelengths. Thus, in a VCSEL, returning light is highly likely to interfere with the light inside of the resonator, and therefore, a noise is highly likely to be produced.
For example, as illustrated in FIG. 44, in an optical system including a coupling lens for converting divergent light output from the VCSEL into parallel rays, and an aperture member for collimating the parallel rays, if the surface of the aperture member is not sufficiently processed to reduce the reflectance thereof (e.g., anti-reflection painting), weak light returns to the VCSEL to produce a noise.
To reduce the effect of the returning light in a VCSEL, it is effective to dispose a quarter-wave plate on the optical path of the light beams output from the VCSEL.
For example, in the manner illustrated in FIG. 45, when a quarter-wave plate is disposed between the coupling lens and the aperture member illustrated in FIG. 44, linearly polarized light output from the VCSEL (linearly polarized light oscillating in the vertical direction on the surface of the drawing in the example illustrated in FIG. 45) is converted into circularly polarized light after passing through the quarter-wave plate. The weak circularly polarized light reflected on the aperture member passes through the quarter-wave plate again, and is converted into linearly polarized light oscillating in a direction perpendicular to the direction of the oscillation of the linearly polarized light output from the VCSEL (linearly polarized light oscillating in the direction perpendicular to the surface of the drawing in the example illustrated in FIG. 45).
In this example, the returning light does not interfere with the light inside of the resonator even if the returning light enters the VCSEL. Thus, no noise is generated.
In this manner, use of a quarter-wave plate can reduce the sensitivity of the VCSEL to the returning light.
However, when a quarter-wave plate is used, the circularly polarized light becomes incident on the plastic-molded scanning lens, and that will worsen the problem of the birefringence. This is because the birefringence changes the degree of polarization of circularly polarized light more prominently than linearly polarized light. A “degree of polarization” herein means the optical intensity of a component oscillating in the main-scanning direction, the optical intensity of a component oscillating in the sub-scanning direction, and the rotation of elliptically polarized light along the long axis in a polarization.
For example, FIGS. 46A and 46B indicate a polarization of linearly polarized light oscillating in the sub-scanning direction after passing through a birefringent medium having a principal axis at 10 degrees and a phase difference of 0.1λ, a polarization of the same light after passing through a birefringent medium having a principal axis at 10 degrees and a phase difference of 0.2λ, a polarization of the same light after passing through a birefringent medium having a principal axis at 20 degrees and a phase difference of 0.1λ, and a polarization of the same light after passing through a birefringent medium having a principal axis at 20 degrees and a phase difference of 0.2λ.
Furthermore, FIGS. 47A and 47B indicate a polarization of circularly polarized light after passing through a birefringent medium having a principal axis at 10 degrees and a phase difference of 0.1λ, a polarization of the same light after passing through a birefringent medium having a principal axis at 10 degrees and a phase difference of 0.2λ, a polarization of the same light after passing through a birefringent medium having a principal axis at 20 degrees and a phase difference of 0.1λ, and a polarization of the same light after passing through a birefringent medium having a principal axis at 20 degrees and a phase difference of 0.2λ.
To calculate the polarizations, the Jones vector and the Jones matrix are used (see “Kogaku no kiso” (Basics of Optics), First Edition, Second Print, Corona Publishing Co., Ltd., pp. 145 to 149).
As may be clear when FIGS. 46B and 47B are compared, the polarization of the circularly polarized light changes more prominently than the linearly polarized light, after passing through a birefringent medium having the same principal axis and the same phase difference.
For example, to compare amounts of change as a ratio (Ix:Iy) between the square of Ex (the amplitude in an x direction) that is an optical intensity Ix in the x direction and the square of Ey (the amplitude in a y direction) that is an optical intensity Iy in the y direction, the linearly polarized light that is incident light at Ix:Iy=1:0 only changes to 0.99:0.01 after passing through the birefringent medium having the principal axis of 10 degrees and a phase difference of 0.1λ, for example. On the contrary, the circularly polarized light that is incident light at Ix:Iy=0.5:0.5 changes to the light at 0.60:0.40. It is clearly understandable, also from this numerical comparison, that circularly polarized light is more sensitive to a birefringence.
Light is more sensitive to a polarization change caused by a birefringence, which means that the reflectance of the light beams reflected on the folding mirror deviates greatly from the ideal after passing through the plastic-molded scanning lens.
If the reflectance deviates greatly from the ideal, utilization efficiency of the light on the target surface varies greatly depending on positions on the target surface in the main-scanning direction. Such variation might cause variations of amounts of light to exceed a correctable range (normally, 10 percent or so) of so-called shading correction in the main-scanning direction for controlling the optical output of a light source to make the amount of light uniform in the main-scanning direction. Inability of correcting such variation results in an uneven density in an output image.